Host Associated Differentiation (HAD) is the formation of genetically distinct host associated populations. One of the genotypic signatures of HAD is that populations exhibit stronger differentiation by host plant species than by geography. HAD, as a mechanism promoting ecological speciation, has been invoked to explain the enormous diversity of phytophagous insects. Individual case studies of HAD are interesting in their own right, but by themselves they are unable to provide evidence of specific herbivore traits that might promote it. Two such proposed traits are endophagy and parthenogenesis. The well-characterized Solidago host plant system has many shared endophages but few shared parthenogens. Thus, a new model system with multiple shared parthenogens is needed. We introduce such a system: pecan and water hickory, both trees in the genus Carya. To date we have recorded 29 insect species shared by these two hosts, including six parthenogens. We sampled seven species of shared herbivores on pecan and water hickory across a four county area in East Texas. We then obtained AFLP fingerprints and analyzed them using Bayesian clustering methods to infer the presence/absence of HAD in each herbivore species. The null hypothesis for each species was panmixia while the alternative hypotheses were either isolation by host-plant species (HAD) or isolation by distance.

Results/Conclusions

For each insect species, we obtained between 72 and 353 AFLP loci excluding private alleles. We found that HAD was present in four out of seven species tested. Species showing HAD were: yellow pecan aphid, black margined aphid, pecan leaf phylloxera, and pecan stem phylloxera. HAD was not present in black pecan aphid, pecan bud moth, and southern pecan leaf Phylloxera. No isolation by distance was detected for any of the species. All aphids are parthenogens and all Phylloxera species are both parthenogenetic and endophagous. Thus, four of six parthenogens and two of four endophages showed HAD when feeding on pecan and water hickory in our study area. This suggests that both endophagy and parthenogenesis may promote HAD. In addition, we stress the importance of the Carya system as a parthenogen rich contrast to the already well characterized Solidago system.